Assigning Surface Properties for General Contact in Abaqus/Explicit

By default, the nodal thickness for surfaces based on shell, membrane, or rigid elements equals the minimum original thickness of the surrounding elements (see Figure 1 and Table 1). If a node is shared by shell and beam elements, the contact thickness that takes precedence is the one derived from the shell element. To account for thick beams that are colocated with shell edges, the beam elements must be attached to the shell edges using a tie constraint (see Mesh Tie Constraints).

Figure 1. Continuous variation of surface thickness across facet boundaries.

The surface thickness within a facet is interpolated from the nodal values; the interpolated surface thickness never extends past the specified element or nodal thickness, which may be significant with respect to initial overclosures. The default nodal surface thickness is zero for regions of a surface based on solid elements. If a spatially varying nodal thickness is defined for the underlying elements (see Nodal Thicknesses), the nodal surface thickness may not correspond exactly to the specified nodal thickness (see node 4 in Figure 2 and Table 2).

Figure 2. Small discrepancy between the nodal surface thickness and the specified nodal thickness.

The nodal surface thickness distribution will tend to be more diffuse than the specified nodal thickness distribution (because the specified nodal thicknesses are averaged to compute the element thicknesses, and the minimum of the surrounding element thicknesses is the nodal surface thickness).

SURFACE PROPERTY ASSIGNMENT, PROPERTY=THICKNESS
surface or material, ORIGINAL (default), , SURFACE (default) or MATERIAL

If you specify that the decreasing parent element thickness should be used, only decreases in the parent element thickness are reflected in the contact surface thickness; if the parent element thickness actually increases during the analysis, the contact thickness will remain constant.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=THICKNESS
surface or material, THINNING, , SURFACE (default) or MATERIAL

You can directly specify the surface thickness value.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=THICKNESS
surface or material, value, , SURFACE (default) or MATERIAL

You can apply a scale factor to any value of the surface thickness. For example, if you specify that the decreasing parent element thickness should be used for surf1 and apply a scale factor of 0.5, a value of one half the decreasing parent element thickness will be used for surf1 when it is involved in a general contact interaction (all other surfaces included in the general contact domain will use the default original parent element thickness). Scaling the surface thickness in this way can be used to avoid initial overclosures in some situations. Abaqus/Explicit will automatically adjust surface positions to resolve initial overclosures (see Contact Initialization for General Contact in Abaqus/Explicit). However, if nodal position adjustments are undesirable (for example, if they would introduce an imperfection in an otherwise flat part, resulting in an unrealistic buckling mode), you may prefer to reduce the surface thickness and avoid the overclosures entirely.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=THICKNESS
surface or material, value or label, scale_factor, SURFACE (default) or MATERIAL

Optionally, Abaqus/Explicit can establish a fixed set of active feature edges for contact based on original feature angles. If no feature angle thresholds are specified explicitly, the list of active feature edges matches the default initially active feature edges for dynamically applied criteria (30° for primary feature edges and 20° for secondary feature edges), but this list is not updated during the simulation. This option is not well suited for common scenarios involving significant deformation during a simulation. However, using a fixed set of active feature edges can save computational time for simulations involving small deformation.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material,,,, ORIGINAL,, SURFACE (default) or MATERIAL

You can limit feature edges for edge-to-edge contact to perimeter edges and contact edges of beams and trusses. Perimeter edges occur on “physical” perimeters of shell elements and on “artificial” edges that occur when a subset of exposed facets on a body are included in the general contact domain. When structural elements share nodes with continuum elements, the perimeter edges are not activated on the structural elements because the criterion to designate them as such is no longer satisfied.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, PERIMETER EDGES (default),,,,, SURFACE (default) or MATERIAL

You can choose particular feature edges on surface, structural, and rigid elements to be activated in domain. A surface containing a list of element labels and edge identifiers (see “Defining edge-based surfaces” in Element-Based Surface Definition) is used to specify the edges to activate.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, PICKED EDGES,,,,, SURFACE (default) or MATERIAL

You can choose to activate all edges each increment in a given surface in the general contact domain. However, this option degrades performance.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, ALL EDGES,,, ORIGINAL,, SURFACE (default) or MATERIAL

You can choose to deactivate all feature edges (including perimeter edges) in the general contact domain. This option does not deactivate “contact edges” associated with beam and truss elements.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, NO FEATURE EDGES,,,,, SURFACE (default) or MATERIAL

If you specify a cutoff feature angle as the feature edge criteria, perimeter edges and geometric edges with feature angles greater than or equal to the specified angle are activated in the general contact domain. By default, the feature angle thresholds are applied dynamically throughout the simulation. Optionally, you can specify that the feature angle thresholds are applied only once at the beginning of the analysis. As described previously, you can activate additional feature edges if required.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, feature_angle_value,,,,, SURFACE (default) or MATERIAL

You can assign a different feature edge criteria to different regions of the general contact domain. For example, Table 3 shows the input that could be used to specify that none of the feature edges of surf1, only perimeter edges of surf2, and perimeter edges and feature edges of surf3 with a feature angle greater than 30° should be considered for edge-to-edge contact.

To reduce computational cost in certain situations, it may be desirable to specify two feature angle criteria for a given surface. Edges satisfying the more restrictive criteria are considered primary feature edges, and edges satisfying the less restrictive criteria only are considered secondary feature edges. If primary and secondary feature edge criteria are in effect, Abaqus/Explicit enforces edge-to-edge contact between primary feature edges and between primary feature edges and secondary feature edges only. Edge-to-edge contact is not enforced between secondary feature edges. This ensures that interpenetrations are avoided at locations where there are “true” edges in the model, without the need to activate primary feature edges at locations where the gradients in the surface normals are only moderate. A judicious choice of criteria for selecting primary and secondary feature edges can lead to significant savings in computational costs.

Secondary feature edges can be selected for a surface by specifying a secondary feature edge criterion in addition to the criterion used to select the primary feature edges for that surface. If the secondary feature edge criterion is omitted, only primary feature edges are activated for the surface. Allowable criteria for secondary feature edges are:

• all edges that have not been selected as primary feature edges;

• all picked edges that have not been selected as primary feature edges;

• all perimeter edges that have not been selected as primary feature edges; and

• all edges with a feature angle greater than a specified cutoff angle value that have not been selected as primary feature edges.

The allowable values for the secondary feature edge criterion permit possible combinations of criteria for primary feature edges and secondary feature edges, shown in Table 4.

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, primary feature edge criterion, ALL REMAINING EDGES,,,, SURFACE (default) or MATERIAL

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, primary feature edge criterion, PICKED EDGES,,,, SURFACE (default) or MATERIAL

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, primary feature edge criterion, PERIMETER EDGES,,,, SURFACE (default) or MATERIAL

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, primary feature edge criterion, feature_angle_value,,,, SURFACE (default) or MATERIAL

SURFACE PROPERTY ASSIGNMENT, PROPERTY=FEATURE EDGE CRITERIA
surface or material, NO FEATURE EDGES, secondary feature edge criterion,,,, SURFACE (default) or MATERIAL

The feature angle is the angle formed between the normals of the two facets connected to an edge. By default, the angles between facets are based on the initial configuration. However, the most efficient approach for accurately resolving contact is often to apply the feature edge criteria to the current configuration. In this case the edges that are eligible for edge-to-edge contact evolve during the simulation.

A negative angle will result at concave meetings of facets; therefore, these edges are not included in the contact domain if the feature edge criteria is based on a cutoff feature angle. Figure 6 shows some examples of how the feature angle is calculated for different edges.

Figure 6. Calculating the feature angle.

The feature angle for edge A is 90° (the angle between $n_1$ and $n_2$ ); the feature angle for edge B is −25° (the angle between $n_2$ and $n_3$ ). Edge C forms a T-intersection with three facets (shown in two dimensions in Figure 7); its feature angles are 0°, −90°, and −90°.

Figure 7. Feature angles for a T-intersection.

Perimeter edges (for example, edge D in Figure 6) can be thought of as a special type of feature edge where the feature angle is 180°.

The sign of the feature angle is considered when determining whether or not a geometric feature edge should be activated in the general contact domain. For example, if a cutoff feature angle of 20° were specified, edge A would be activated as a feature edge in the contact model (90° > 20°) but edges B and C would not be activated: −25° < 20° and 0° (the maximum feature angle for edge C) < 20°.

Figure 8 illustrates further how the feature angle is used to determine which geometric feature edges should be activated in the general contact domain.

Figure 8. Feature edges activated in the general contact domain for a cutoff feature angle of 20°.

The table to the right of the figure lists the feature angle values for various edges in the model. Edges connected to more than two facets, as well as edges connected to two shell facets, have more than one corresponding feature angle. The largest feature angle at an edge is compared to the specified cutoff feature angle. For example, if a cutoff feature angle of 20° were specified, edges A, D, and E would be considered feature edges, while edges B, C, and F would be ignored for edge-to-edge contact.

The contact output variable CEDGEACTIVE is available to identify throughout the analysis if an edge is active as a primary edge, active as a secondary edge, or has been deactivated by the contact domain.

The contact smoothing technique assumes that the initial locations of the surface nodes lie on the true initial surface geometry, with the exception of midedge nodes of C3D10M elements. This smoothing technique remains effective even if the midedge nodes of C3D10M elements do not lie on the true initial geometry.

The effects of contact smoothing tend to be most significant for analyses involving small deformation, and the smoothing technique works well for cases involving large relative motion between the surfaces. For analyses with large deformation this smoothing technique typically has an insignificant effect on the solution. However, in some cases—especially where the underlying elements can fail—the smoothing can degrade the solution accuracy after large deformation.

The impact of contact surface smoothing can be demonstrated by a simple model of contact between concentric cylinders with a small clearance between them. With a matched mesh as shown in Figure 9 there are no initial overclosures; therefore, there are no initial strain-free initial displacement adjustments. However, if the inner cylinder is rotated, the cylinders develop stresses (see Figure 10) as contact is detected due to the linear faceted representation of the main surface. This behavior is improved when the circumferential smoothing technique is applied to the contacting surfaces of the two cylinders.

Figure 9. Concentric cylinders with matched mesh.

Figure 10. Stresses as cylinder rotates.